Kody Brown, center, poses with his wives (from left) Robyn, Christine, Meri, and Janelle. The Browns star in the TLC series "Sister Wives."

"Conservatives foresaw polygamy ruling," a story in the Washington Times was headlined last week. It cited the "we-told-you-so" reactions of several longtime opponents of same-sex marriage, who have long argued that the radical transformation of marriage wouldn't end with gay wedlock.

"Sometimes I hate it when what I predict comes true," former Senator Rick Santorum tweeted after US District Judge Clark Waddoups held in favor of Kody Brown and his four wives, who had challenged Utah's ban on polygamy as a violation of their right to privacy. The judge's ruling still leaves plural marriage technically illegal in Utah, but only "in the literal sense" of having two or more marriage licenses. Otherwise, polygamy has now been effectively decriminalized in Utah - a state admitted to the union on the condition that it forever ban the practice of polygamy.

Not every change in law or policy is the first step down a slippery slope to a more drastic or unwelcome change. No one imagines that requiring a license for a pet dog today means you'll have to get a license for a pet fish tomorrow. But when a longstanding consensus on the meaning of a bedrock societal institution is altered - especially one as entwined with moral values and social attitudes as marriage - it is na´ve or disingenuous to claim that even more extreme changes won't follow.

Yet time and again, advocates of same-sex marriage have pooh-poohed the warning that if marriage is redefined so that the sex of the spouses is irrelevant, it can be further redefined so that the number of spouses, or the family relationship of the spouses, is also irrelevant.

To be sure, many gay-marriage proponents oppose polygamy. Andrew Sullivan has long said society's rule should be: "We won't let you legally marry more than one person, but we encourage you to marry one." But if the essence of marriage is a right to marry whomever you love, what reasonable grounds are left for saying no to polygamists like Kody Brown and his multiple wives? Or to any other union of consenting adults?

When the Massachusetts Supreme Judicial Court ruled in 2003 that same-sex couples had a constitutional right to marry, the majority opinion dismissed such concerns. "Plaintiffs seek only to be married, not to undermine the institution of civil marriage," Chief Justice Margaret Marshall wrote. "They do not attack the binary nature of marriage, the consanguinity provisions, or any of the other gate-keeping provisions of the marriage licensing law."

But ideas have consequences - often, unintended consequences. That is particularly true in a legal system that places so much emphasis on precedent and analogy.

In 1989, as Massachusetts lawmakers were about to enact a law barring discrimination on the basis of sexual orientation, the Boston Globe assured its readers that the bill wouldn't legalize gay marriage or confer on same-sex couples the right to marriage-like civil unions. "Nor does passage of the bill put Massachusetts on a 'slippery slope' toward such rights," it editorialized. Yet when the SJC ruled that same-sex couples could not be barred from marrying, as UCLA law professor Eugene Volokh has noted, "part of its reasoning rested on the legislature's decision to ban sexual orientation discrimination." That slope was slippery, after all.

This happens frequently in the law. The US Supreme Court ruled in the 1965 case of Griswold v. Connecticut that married couples had a constitutional right to privacy that encompassed the freedom to use contraceptives. During oral argument, Justice Hugo Black wondered whether the court's logic could lead to "invalidat[ing] all laws that punish people for bringing about abortions." The suggestion was dismissed, as slippery-slope arguments so often are, as groundless.

"Yet only seven years after Griswold, in Eisenstadt v. Baird, the Court relied on Griswold to hold that unmarried couples have a right to use contraceptives," Volokh points out. "The following year, the Court used Griswold and Baird as the foundations for recognizing a right to abortion" in Roe v. Wade -just as Black had warned.

Supporters of same-sex marriage - or of any other controversial policy - should be honest enough to acknowledge that it may pave the way to future changes that they don't support. What they shouldn't do is sneer that slippery-slope arguments are inherently baseless. As a judge in Utah has just reminded us, X can indeed lead to Y.

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